Soft X-Ray Background Data Sets and Other Resources at the HEASARC
and Elsewhere

The last mission to survey the sky in the soft X-ray band was ROSAT, which
conducted an all-sky survey from 1990 July 30 to 1991 January 25, and from
1991 August 3 to 13.

The next experiment to carry out a soft X-ray all-sky survey will be eROSITA. eROSITA will be the
primary instrument on-board the Russian
"Spectrum-Roentgen-Gamma" (SRG) spacecraft that will be launched from Baikonur
in 2018 and placed in an L2 orbit. It will perform the first imaging all-sky
survey in the soft and medium energy X-ray range (0.3 - 10 keV) with an
unprecedented spectral (138 eV at 6 keV) and angular (16 arcseconds on-axis)
resolution, and sensitivity, with a grasp (effective area * FOV on sky) that
gets as high as 1000 cm2 deg2 at 1.5 keV.

MAST EUVE
ISM Hydrogen Column Density Search Tool: This tool is no longer
supported, sorry! [Calculates the Galactic H I
column density in a given direction and to the user-specified distance].

SkyView:
Contains more than a dozen soft and hard X-ray data sets that the user
can replot if so desired.

All-Sky Soft X-Ray Background Data Sets at the HEASARC

The HEASARC has the following data sets which contain measurements of the
X-ray background all-sky intensity and/or spectra obtained with different
experiments, listed in inverse chronological order (most recent first):

ROSAT All-Sky Survey (RASS)

These maps of the diffuse X-ray background in the
0.11 - 2.04 keV energy range ('ROSAT Survey Diffuse Background
Maps, Paper II', S. L. Snowden, R. Egger, M. J. Freyberg, D. McCammon, P. P.
Plucinsky, W. T. Sanders, J. H. M. M. Schmitt, J. Trümper, and W. Voges,
ApJ, Vol. 485 (1997), pp. 125-135) are used by the
HEASARC's X-Ray Background Tool (XRBT). This tool calculates for a
specified astronomical position and either a circle with a specified radius
or an annulus with specified inner and outer radii centered on this position
the average X-ray background count rate and statistical uncertainty in each
of the six standard bands of the ROSAT All-Sky Survey diffuse background maps
(R1, R2, R4, R5, R6, R7). In addition, the average count rates are given for
the combined 1/4 keV (R1 + R2), 3/4 keV (R4 + R5), and 1.5 keV (R6 + R7)
bands. (The count rates for the combined bands are simply the sum of the
count rates for each component band, and the uncertainty in the combined
band count rate is the square root of the sum of the squares of the
uncertainties of each component band). A full description of the use
of these maps is given in the XRBT help
documentation.

Diffuse X-Ray Spectrometer (DXS)

The DXS experiment was flown on the space shuttle Endeavour from January 13
through 19, 1993. DXS was developed by the University of Wisconsin and its PI
was Dr. Wilt Sanders. The HEASARC has,
the so-called DXS level 1 reduced data products stored in
this directory in the
form of gzipped FITS files.
The naming convention of the files is of the form "orbitNN_TYPE_SIDE.fits.gz",
where NN = 2-digit orbit number in the range 1 to 91, TYPE = 3 or 4 character
type of data file, out of the set:

and SIDE = 1-character indicator of the DXS instrument that the data are
from:

p - Port instrument
s - Starboard instrument

and the final .gz indicates that the file has been gzipped. These files can
be read using standard FITS readers such as fdump.

The HEASARC also has the DXS spectral products in
this directory
which were derived from a 2004 reprocessing
of the raw telemetry. These spectra are trivially different that
those presented in
Sanders et al. (2001, ApJ, 554, 694) because of minor bug
fixes in
the processing software. See Sanders et al. (2001) for a detailed
discussion of the DXS spectra.
Morgenthaler (1998, PhD thesis) has detailed how the spectra and response
matrices
were derived. The original spectra, response and arf files supplied to the
HEASARC were modified in April 2009 in order to be compatible
with xspec12 and with the FITS standards for spectra, response and arf.

In brief, the clean* spectra represent spectra from different regions
along the galactic equator, as indicated by the GAL_LON bin in the
FITS headers (see also Sanders et al. figs. 8 and 10). The file
clean_Allsky_p.flat contains the summation of all of the sky spectra
and clean_HISM_p.flat contains the summation of the regions with the
lowest count rate (Crux, Pup, Aur). The file clean_back_p.flat
contains the spectrum of the instrument background and the dxsmodel*
files contain the instrument response matrix in standard OGIP form.

The total instrument count rate varied in time over the duration of
the flight, with the first half having a higher count rate than the
second half (see Sanders et al. fig. 9). The files prepended with
"1_" and "2_" contain the spectra from the first and second half of
the flight, respectively.

The SAS-3 low-energy detector

This directory
contains the all-sky image from the SAS-3 low-energy X-ray
detector in the C band (0.10-0.28 keV in FITS format. These data were
discussed in the paper by Marshall and Clark (1984, ApJ, 287, 633-652: see
the ADS abstract appended below). The file sas.fits contains the all-sky
intensity map in Galactic coordinates in an Aitoff projection centered
on the Galactic Center.

Abstract: The results of a survey of the soft X-ray sky in the C band (0.10 -
0.28 keV) are reported. The observations were carried out using two independent
flow proportional counters on board the SAS 3 X-ray satellite which had a
total angular resolution of 2.9 deg FWHM, and a total exposure of 22,000 cm^2
s sr. It is found that C band counting rates were generally inversely
correlated with the column density of the neutral hydrogen on all
angular scales down to the lowest angular resolution of the detectors. In the
region 90-180 degrees Galactic longitude and 0-90 degrees Galactic
latitude, the relation between C-band rates and the column densities of
neutral hydrogen was fitted with a residual rms deviation of less than 13%
by a two-component numerical model of the X-ray background. For the apparent
attenuation column density, a value of 2.7 x 10^20 per sq cm was obtained.
On the basis of a computer simulation of the SAS 3 data, it is shown that
the observed clumping of interstellar matter was consistent with the
magnitude of spatial fluctuations in the C-band map. When the background
rates were subtracted from the survey map, the subsequent map showed
foreground emission and absorption features with improved sensitivity and
clarity. A series of computer-generated maps incorporating the SAS 3 data
is given in an appendix.

The HEAO 1-A2 experiment

This directory
contains the spectra (and response matrices) of the X-ray
background obtained with the HEAO1 A2 experiment (Marshall et al., 1980, ApJ,
235, 4: see below) in mid-1977.
The spectra and responses are all in FITS formats:

Abstract: The spectrum of the extragalactic diffuse X-ray background has been
measured with the GSFC Cosmic X-ray Experiment on HEAO 1 for regions of the sky
away from known point sources and more than 20 degrees from the galactic plane.
A total exposure of 80 m^2-s-sr is available at present. Free-free emission
from an optically thin plasma of 40 +/- 5 keV provides an excellent description
of the observed spectrum from 3 to 50 keV. This spectral shape is confirmed by
measurements from five separate layers of three independent detectors. With an
estimated absolute precision of about 10%, the intensity of the emission at
10 keV is 3.2 keV/keV/cm^2/s/sr, a value consistent with the average of
previously reported spectra. A uniform hot intergalactic medium of
approximately 36% of the closure density of the universe would produce such a
flux, although nonuniform models indicating less total matter are probably
more realistic.

Abstract: Results of a study of the diffuse soft-X-ray background as observed
by the low-energy detectors of the A-2 experiment aboard the HEAO 1 satellite
are reported. The observed sky intensities are presented as maps of the diffuse
X-ray background sky in several energy bands covering the energy range 0.15-2.8
keV. It is found that the soft X-ray diffuse background (SXDB) between 1.5 and
2.8 keV, assuming a power law form with photon number index 1.4, has a
normalization constant of 10.5 +/- 1.0 photons/cm^2/s/sr/keV. Below 1.5 keV,
the spectrum of the SXDB exceeds the extrapolation of this power law. The
low-energy excess for the north ecliptic pole (NEP) can be fitted with
emission from a two-temperature equilibrium plasma model with the temperatures
given by log T1 = 6.16 and log T2 = 6.33. It is found that this model is able
to account for the spectrum below 1 keV, but fails to yield the observed
Galactic latitude variation.

The University of Wisconsin experiments flown on 10 sounding rockets in the
period from 1972 to 1980

A compendium of documentation and data from the
University of Wisconsin
experiments flown on 10 sounding rockets over the period from 1972 to 1980 is
available in the
CDS
Archive.

In this
HEASARC directory, we have the all-sky images created from these
data.
All of the files are in FITS format. These data were discussed in the paper
by McCammon et al. (1983, ApJ, 269, 107-135: see the ADS abstract appended
below). The 52 files comprise 26 pairs of different visualizations of the
soft X-ray diffuse background, corresponding to different energy bands and
projections, the rat.fits files containing the intensity maps, and the
sig.fits files containing the uncertainty maps. The first part of the file
names codes the X-ray band to which it corresponds as follows:

Abstract: Maps of the diffuse X-ray background intensity covering essentially
the entire sky with approximately 7 degree spatial resolution are presented
for seven energy bands. The data were obtained on a series of ten sounding
rocket flights conducted over a seven-year period. The different nature of
the spatial distributions in different bands implies at least three distinct
origins for the diffuse X-rays, none of which is well-understood. At energies
of approximately 2000 eV, an isotropic and presumably extragalactic 500 and
1000 eV, an origin which is at least partially galactic seems called for.
At energies 284 eV, the observed intensity is anticorrelated with neutral
hydrogen column density, but we find it unlikely that this anticorrelation
is simply due to absorption of an extragalactic or halo source. Previously
announced in STAR as N83-20892.